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Mudassir Ali

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Quantum gravity aims to unite the concepts of quantum theory with those of general relativity. In other words, the attempt is made to quantize the gravitational field. The associated gravitational field quantum is called graviton in quantum field theories (QFT). In the Loop quantum gravity (LQG) one rather distances oneself from the concept of graviton and calls the quanta of the theory loops.

In the general relativity, space-time is a deformable, four-dimensional, continuous structure, a space-time continuum, which is curved by forms of energy such as mass. The classical theories do not provide for principles such as discrete physical quantities (quanta). Spacetime appears sufficiently smooth, continuous, and has at best ‘holes’ in the form of singularities of curvature (the black holes are an example of this). Now, however, quantum theory and in particular the Planck scale derived from it with the general relativity imply that with very strong gravitational fields on very small length scales a grain size of space-time must appear. The length scale, where the grain occurs, is in the range of the Planck length, thus on only 10^-33 cm. This is the smallest unit of length known to physicists. Granulation requires a theory that goes beyond general relativity and takes into account the quantum character of spacetime: quantum gravity. Therefore, the time is quantized in the fundamental units of Planck’s time, i.e. in ‘time packets’ of 10^-43 seconds. Thus the time in the LQG is not the proverbial, continuous flow, but rather comparable to the ticking of a clock.